1. Field of the Invention
The invention relates to a method and to a device for imaging with high geometrical resolution on a microsatellite.
2. Discussion of the Prior Art
In order to obtain high geometrical resolution simultaneously with a large capture angle, linear CCD array sensors are used in the camera systems for satellite applications. Together with the increase in the geometrical resolution for a given orbital altitude, it is necessary to accept the reduction in the energy incident on a picture element. When increasing the focal length of the camera and at the same time reducing the field of view of the pixels, there are two possible solutions in order to compensate for the reduced input energy on the detector:
1. Increasing the aperture of the objective
2. Lengthening the integration time.
The linear increase in the objective diameter, which is needed when increasing the focal length, is accompanied by a cubic increase in mass and volume of the objective. The reduction in the integration time when reducing the ground pixel size results in a further increase of the objective diameter. This first possible solution is inappropriate for microsatellite application since these must have only a very small mass.
The integration time tint is chosen, in linear CCD array cameras working in push-broom mode, to be less than or at most equal to the time tdwell which is needed for shifting the projection of a CCD picture element onto the surface of the earth by one pixel dimension tintxe2x89xa6tdwell, i.e. ground pixel blurxe2x89xa61 pixel. If the radiation energy reaching the CCD element for a given objective/filter arrangement is not enough to obtain a particular signal/noise ratio, then the ground track projection of the linear CCD arrays needs to be increased in the direction of motion by adding other linear CCD arrays arranged in parallel with the first linear CCD array. The signal/noise ratio can, with a fixed satellite platform/objective/focal plane arrangement, be increased using the TDI principle (Time Delay and Integration; Riichi Nagura: SN Improvement Ratio by Time Delay and Integration for High-Resolution Earth Observatorium System. Electronics and Communication in Japan, Part 1, Vol. 78, 1995, No. 3, pp 74-84). In the TDI principle, with n linear CCD arrays, the satellite movement and the blurring of the ground trace which this causes are compensated for by n-tuple sampling of a linear array projection onto the surface of the earth and subsequent accumulation of the n linear array information items in each case shifted by one tdwell. To obtain this effect, the n-tuple data rates of the detector arrangement need to be processed and compressed. The outlay needed for this with the electronic circuits and the electrical energy needed prohibit applications on microsatellites.
German reference DE 195 02 045 discloses a device for imaging with high geometrical resolution on satellites. In this device the camera platform is tracked using a tracking system counter to the velocity vector of the satellite or the focal plane using piezoelectric actuators, in such a way as to compensate for the blurring effect of the satellite movement over the n-tuple time tdwell. The mechanical impulses generated by the movement of the masses, that is to say the platform and the camera or the focal plane with the electronic components for CCD drive and read-out, the components for thermal stabilization, as well as the cabling needed for this, are not negligible in the case of the small mass inherent in the microsatellite and make it difficult to obtain a high geometrical resolution. Thus, it is not possible to resort to such compensating measures for applications on a microsatellite.
The object of the invention is therefore to provide a method and device for imaging with high geometrical resolution on a microsatellite.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in a device for imaging with high geometrical resolution on a microsatellite, which device comprises an objective, a focal plane, an extended optical detector fitted on the focal plane, and moveable intermediate optics arranged between the objective and the focal plane. The objective and the focal plane are fastened rigidly to the microsatellite. By virtue of the rigid arrangement of the objective and the focal plane, between which intermediate optics that can be moved using a setting and resetting device are arranged, bathe energetic action time on the optical detector for each picture element is lengthened. The picture elements are sampled once after complete tracking. Because of the low mass of the intermediate optics in relation to the objective and the focal plane, the mechanical pulses which are generated are negligible, while, because of the single sampling, the outlay for the data processing is not increased, unlike with the TDI method.
In another embodiment of the invention, the intermediate optics comprise a beam deflecting instrument, so that the focal plane can be arranged substantially independently of the objective. This makes it possible for the microsatellite to have a compact structure. The beam deflecting device may, for example, consist of optical fibers or prisms, although it is preferably formed by at least one mirror.
In a further embodiment, the intermediate optics comprise an optical field adapter which is arranged between the beam deflecting instrument and the focal plane and is designed, for example, as a converging lens. In order to produce the tracking movement, either the mirror arrangement or the field adapter are then designed to be mobile, in order to balance the microsatellite""s own movement.
The extended optical detector is, for example, designed as a CCD matrix. However, since CCD matrices are very expensive, the extended optical detector is preferably formed by n linear CCD arrays arranged without gaps between them.
The setting and resetting instrument is, for example, designed as a piezoelectric actuator or as a stepper motor. A return spring may optionally be allocated to the setting and resulting instrument in order to move the intermediate optics more rapidly back into the starting position. Preferably, however, the intermediate optics are at least partially integrated with the setting and resetting instrument. To that end, the mirror and the setting/resetting instrument are produced using suitable selective etching processes as an integrated rotatable micromirror with allocated actuators. The advantage of these components produced using microsystem technology is their very small volume and low weight.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.